The formation of pollutant nitrogen oxides is known to increase with increasing combustion or flame temperature.
In the usual known combustion systems of the so-called diffusion type in which the fuel is injected into a combustion chamber surrounded by an interspace containing pressurized air flowing counter-currently to the stream of combustion products and comprising holes for the combustion air, small intermediate apertures distributed over the entire chamber surface for the chamber cooling air and holes for the dilution air which reduces the temperature of the combustion products to a level acceptable by the turbine, the fuel burns an air quantity always equal to the stoichiometric value and hence always with a high energy concentration and a high temperature whatever the excess air present, and hence without consequent flame stability problems even under low load, but with evident high pollutant emission. In order to reduce this pollutant emission, use is generally made of combustion systems with pre-mixing in which upstream of the combustion chamber, from which it is separated by a constriction and which is also surrounded by said interspace containing pressurized air, there is provided a pre-mixing chamber into which both the fuel and said combustion air are fed, these mixing at low temperature to substantially dilute the fuel before reaching the combustion chamber, so that said combustion is no longer stoichiometric but instead takes place with an excess of air and hence at a lower temperature.
It has now been found that to achieve low pollutant emission of nitrogen oxides and carbon monoxide together with good flame stability it is necessary to maintain the combustion air/fuel ratio around an optimum value corresponding to an air excess of between 1.5 and 2 times the stoichiometric value, this being achievable with pre-mixing combustion systems at all turbine loading levels.
In this respect, whereas the air flow fed to said combustion systems generally by an axial compressor remains substantially constant, the fuel quantity has to be varied continuously on the basis of the turbine loading, so that if said optimum air excess is achieved at full load, it is no longer achieved when the turbine is used at reduced load, i.e. when using a smaller fuel quantity. In such cases the air excess can reach between 4 and 7 times the stoichiometric value, with the consequent danger of the flame extinguishing.
In addition to the said possible extinguishing of the flame, a further drawback of pre-mixing combustion systems is that they easily produce unstable combustion due to the fact that the low energy concentration present makes the flame sensitive to the smallest disturbances, hence producing deleterious pressure pulsation within the combustion chamber.